Latent image intensification



Patented Nov. 13, 1951 2,575,002 LATENT IMAGE INTENSIFICATION Burt H. Carroll, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application July 21, 1948, Serial No. 40,012.

7 Claims. 1

This invention relates to the latensification of photographic images particularly by means of gold salts.

Various methods have been proposed for intensifying the latent image and effectively increasing the sensitivity and speed of photographic emulsions. Of the proposed methods, the use of a noble metal such as gold in the colloidal state has produced noteworthy if somewhat unpredictable and erratic results depending upon the method of preparation of the gold sol and other factors. However, gold salts have not previously been employed with success as latensifiers. Gold salts have been used in photographic emulsion as hypersensitizers as opposed to the present use for latensification where the salt is applied subsequent to exposure and prior to development. Gold salts have also been used in photographic processes in a manner different and for purposes other than employed herein. That is, gold salts have found extensive use in toning baths which are applied to already developed images in accordance with the toning bath art. However, I am not aware that gold salts, particularly those of my invention have been previously employed in the manner and for the purpose herein described.

I have discovered certain gold salts which are easily prepared and which give consistent results when employed for the latensification of photographic images. Therefore, one object of my invention is to provide gold salts which are good latensifiers. Another object is to provide the materials and method of making the effective gold salts. A further object is to provide methods of using the latensifying agents of my invention in enhancing properties of emulsions such as sensitivity.

The objects of my invention are accomplished in part by employing as a latensifying agent an aurous thiocyanate complex salt obtained by reacting an auric salt with an alkali metal thiocyanate.

The gold salts which I use are obtained by adding a solution of a soluble auric salt such as auric chloride, or a complex gold halide such as potassium auric chloride (KAuCh) or auric sulfate, to a hot solution containing an excess of an alkali metal thiocyanate such as sodium or potassium thiocyanates. The resulting aurous thiocyanate salts are complex in structure the nature of which is not known with certainty.

However, I have found that the complex salt is most stable in the presence of excesses of thiocyanate, therefore I employ, in making the gold thiocyanate complex and/or in the latensifying baths, from about 2 to 20 times as much by weight of alkali metal thiocyanate as gold salt.

The gold complex salts of my invention are employed, as previously mentioned, in aqueous solutions which are applied to exposed emulsion layers prior to development, preferably in concentrations equivalent to about 0.0005 to 0.001% auric salt and 0.0025 to 0.005% alkali metal thio cyanate. That is, I may react a solution of about 0.5 to 10% thiocyanate with a solution of about 0.1% auric salt such as auric chloride and for use dilute to the required concentra tion.

Any of a number of well-known types of emulsions may be latensified with my gold salts such as emulsions predominant in silver bromide or silver chloride, preferably the former, for example, the so-called bromo-iodide emulsions. The preference is obvious in as much as one object of latensification processes is to obtain the maximum. sensitivity at a given exposure level and this is provided by bromo-iodide emulsions. Silver chloride emulsions are also less preferred because silver chloride reacts with thiocyanate ion to form the less soluble silver thiocyanate. Also the emulsions may contain the usual optical or chemical sensitizers. In the following table is shown the results of latensifying three different types of photographic emulsions with a gold thiocyanate complex at the concentrations indicated. The tests from which the following tabulation was made were conducted on strips of the films exposed in an Eastman type IIB sensitometer, and after exposure, treatment with the indicated gold thiocyanate bath for about 3 minutes, following which development was carried out under conditions normal for the respective emulsions in a developer of the following composition:

Water, about F. (50 C.) cc 500 p-Methylaminophenol sulfate grams 2.2 Sodium sulfite (desiccated) do 96.0 Hydroquinone do 8.8 Sodium carbonate (desiccated) do 48.0 Potassium bromide do 5.0 Cold water to make liter 1.0

The results of the test are as follows:

Sound Recording Roll Film X-Ray Treatment 101i Speed Gamma Fog 10/i Speed Gamma Fog 10/i Speed Gamma Fog Control 81. 3 1. 56 03 513 0. 78 08 692 1.16 08 0.12% KBr after exposure 67. 5 1. 58 O3 725 0. 98 05 1, 100 0. 61 04 0.12% KBr+0.0005% AuC1a+0.0025% NaGNS after exposure 141 1. 25 03 794 0.92 07 l, 940 0. 61 04 0.12% KBr+0.00l0% AuCl +0.0050% NaCNS after exposure l. 42 04 1, 000 0. 96 07 l, 730 0. 70 05 In the above representative'examples bromide was used in the latensifying bath to maintain the bromide level but this is not essential. The

remarkable increases in sensitivity (lO/i. speed) shown above, obtainable by operating according to my invention, are not b'EHGYSdQtW-bfl: attribut-- able to the presence of unreacted alkali metal thiocyanate, since thiocyanate would be expected only to increase the rate ofdevelopment (gamma) and not speed. Similarly, simple auric and aurous salts such as auric and aurous halides would be expected to give fog and loss of density, In this respect the results produced bythe gold thiocyanate complexes are unexpected. Results of the. order indicated are best obtainable by treatment of the exposed. emulsionsfor short lengths oftime of the order of 2-10 minutes, 2-4 minutes being, preferred. Longer treatment as. for 24 hours results in fog.

It is to be understood that the disclosure herein is by way of example and thatI consider as included in my invention all. modifications and equivalents falling withinv the scope. of the appended. claims.

What I claim is: I

1. The method of intensifying the latent image of a photographic silver halide emulsion layer which comprises treating. said emulsion layer, subsequent to exposure and prior to development, with an aqueous solution obtained by adding a solution of a soluble. auric salt. to a hot solution containing an excess of an alkali metal thiocyanate, and developing the latent image.

2. The method of intensifying the latent image of a silver bromide emulsion layer which comprises treating said emulsion layer, subsequent to exposure and. prior to development, with an aqueous solution obtained by adding a solution of a soluble auric salt to a hot solution containing an excess of an alkali metal thiocyanate, and developing the latentimage.

3. The method of. intensifying; the latent image of a silver chloride emulsion layer which comprises treating said emulsion layer, subsequent to exposure and prior to development, with an aqueous solution obtained by adding a solution of a soluble auricsalt to a hot solution containing an excess of an alkali metal thiocyanate, and developing. the. latent image.

4. The method of intensifying the latent image of a photographic silver halide emulsion layer which comprises treating said emulsion layer, subsequent to exposure and prior to development, with an aqueous solution obtained by adding a solution of a soluble auric salt to a hot solution of from 2 to 20 parts of an alkali metal thiocyanate, and developing the latent image.

5. The method of intensifying the latent image of a photographic silver halide emulsion layer which comprises treating said emulsion, layer, subsequent to exposure andprior to development, with an aqueous solution obtained by adding, a solution of auric chloride to a hot solution of from 2 to 20 parts of an alkali metal thiocyanate, and developing the latent image.

6. The method of intensifying the latent image of a photographic silver halide emulsion layer which comprises treating said emulsion layer, subsequent to exposure and prior to development, with an. aqueous solution obtained by adding a solution of auric chloride to a hot solution of an excess of an alkali metal thiocyanate, and developing the latent image.

'7. The method of intensifying the latent image of aphotographic silver halide emulsion layer which comprises treating said emulsion layer, subsequent to exposure and prior to development, with an aqueous solution obtained by adding a solution of an alkali metal auric chloride to a hot solution of an excess of analkali metal thiocyanate and developing the latent image.

BURT H. CARROLL.

REFERENCES CITED' UNITED STATES PATENTS Name Date Waller et al Apr. 23, 1946 Number: 

